British Transport Police is one of three forces not covered by the chancellor’s pledge.
Photograph: Matt Dunham/AP

Three police forces are at risk of having their budgets cut, despite the chancellor George Osborne’s pledge to protect police spending, the government has admitted. The forces concerned protect the transport system, army barracks and nuclear power infrastructure.

The chancellor sprang a surprise when he promised to safeguard police spending during his spending review last month. But that was labelled a con by Labour on Sunday after the policing minister Mike Penning admitted that the pledge only applied to the 43 police forces funded by the Home Office.

Three others – British transport police, defence police and the civil nuclear constabulary – are afforded no such security because they are funded by other Whitehall departments, as well as industry sources.

The Treasury was unable to say on Sunday whether or not government funding of those three forces would be protected. Furthermore, the Treasury said that the private funding given to them was out of the government’s control.

Labour initially claimed that the police budget promise was, in fact, a real terms spending cut because it only protected current levels of funding. But the Treasury insisted that was not the case.

The spending review document made it clear that the government would “protect overall police spending in real terms over the spending review period, an increase of £900m in cash terms by 2019-20”.

Sunday’s revelation will put further pressure on Osborne over police spending after the home secretary, Theresa May, insisted that, despite the chancellor’s promise to protect their overall budgets, police forces would still have to find savings.

The admission was made in a ministerial reply to a written parliamentary question from Labour’s shadow Treasury minister Rebecca Long-Bailey.

“Two weeks ago, George Osborne promised the British people he would protect the police who protect us – now we know that promise was just another con,” Long-Bailey said on Sunday. “We need a guarantee that the police forces in the frontline against terrorism won’t be in the firing line. The chancellor wasn’t protecting the police; he was just promoting himself.”

A government spokesperson said: “It’s ridiculous to suggest the government is not committed to funding the forces that keep our country safe. At the spending review, we protected overall police spending in real terms with an increase of £900m cash by 2019-20 to maintain strong frontline policing and further strengthen police firearms resource to protect UK citizens. This is on top of an extra £500m extra funding for the counter terrorism budget, to protect the UK from the ongoing threat posed by terrorism.

“Funding for other specialist police forces is not provided by a central government grant and comes from other sources, including industry. We are committed to funding our police forces across the country to protect Britain’s national security.”

The three unprotected forces employ more than 6,300 officers between them – equivalent to some of Britain’s biggest forces.